This proposal describes the US portion of the international program
for study of Deep Ocean Ventilation Through Antarctic Intermediate
Layers (DOVETAIL), a program which has been conceived and organized
by the international Antarctic Zone (iANZONE)
group. The primary goal of iANZONE is to advance our quantitative
knowledge and modeling capability of the seasonal cycle and interannual
variability of the ocean and its sea ice cover, with emphasis
on climate relevant fluxes which couple the Antarctic Zone to
the atmosphere and to the Global Ocean. The first iANZONE field
activity was Ice Station Weddell in 1992, directed at exploration
of the environmental conditions along the western margin of the
Weddell Sea [ISW Group, 1993] and at the formation and spreading
of Antarctic Bottom Water. The second activity was the Antarctic
Zone Flux (ANZFLUX) experiment in 1994, in which heat fluxes within
the winter mixed layer, sea ice and atmospheric boundary layer
were precisely measured [McPhee, 1995]. These two activities
focused upon processes associated with ocean ventilation within
the polar waters.

DOVETAIL builds upon the two preceding iANZONE programs and enters
a second iANZONE phase whose purpose is to better define and understand
the role of Antarctic waters and processes in the global ocean
and climate system. DOVETAIL proposes to focus on escape of the
recently ventilated deep water from the Weddell Sea into the Global
Ocean - the final stage in its role of ventilating deep ocean
waters. The DOVETAIL study region is shown in Figure 1.

Figure 1. Chart of the Weddell-Scotia Confluence region, showing
proposed summer and winter cruise tracks (dashed and solid lines,
respectively, with bold italicized and regular numbers indicating
stations at transect ends) and mooring locations (large dots numbered
1-21). Heavy dashed lines north and south of the South Scotia
Ridge show approximate respective locations of the Scotia and
Weddell frontal systems. Moorings 7-18 are proposed for the US
program, while 1-6 and 19-21 are planned as part of the coordinated
foreign program.

DOVETAIL priorities parallel, and the results will contribute
to, ongoing global change research. The processes responsible
for vertical and horizontal fluxes within the ocean and associated
interaction with the sea ice and atmosphere in polar regions must
be properly represented. The DOVETAIL study region, off the tip
of the Antarctic Peninsula, serves as the primary gateway between
the southern polar waters and the global ocean. This region can
therefore be considered as a "vital" location with respect
to discharge of cold Antarctic Water into the global ocean. Results
from the DOVETAIL experiment will aid in establishing a basis
for long-range monitoring of this critical region, inasmuch as
both the Global Ocean Observing System (GOOS) and the ocean component
of the Global Climate Observation System (GCOS) have been established
by a number of international bodies to provide such monitoring
data.

The overall DOVETAIL goal can be stated as follows:

To understand physical processes in the Weddell-Scotia Confluence
region sufficiently to quantify the ways in which it influences
ventilation of the World Ocean by Weddell Sea water.

As discussed below, the Weddell-Scotia Confluence is thought to
represent a gateway for the most direct and largest contribution
of these Antarctic waters. It is imperative that we understand
the associated physical processes in order that we be able to
assess their sensitivity to changes in regional forcing, hence,
the impact of such changes on Global Ocean ventilation.

1.2. Scientific Background

Approximately 57% of the deep ocean is colder than 2.0=F8C [Gordon,
1991]. The North Atlantic Deep Water, which is the northern hemisphere
source for this deep water, has a characteristic temperature near
or above 2.0C. Therefore most of this deep, cold water mass must
be influenced to some extent through admixture of Antarctic Bottom
Water, the only other available source having temperatures below
2.0C. Locarnini [1994] estimates that 53% of the Antarctic water
which ventilates the deep ocean originates in the Weddell Sea.
It has been estimated [Gordon, 1975; Gordon and Taylor, 1975;
Gordon and Huber, 1990] that a total of about 40 Sv Antarctic
Bottom Water is needed to ventilate the World Ocean, so about
21 Sv must originate from the Weddell. Less than 6 Sv of this
total can reasonably be supplied as topographically controlled
bottom boundary currents which flow north through gaps in the
Scotia Ridge [Fahrbach, 1994; Muench & Gordon, 1995].

It is hypothesized that the remaining 15 Sv of Weddell Sea water
must be transferred north through the Weddell-Scotia Confluence
region. Much of this transfer likely occurs in association with
the Weddell and Scotia fronts via processes which include isopycnal
and diapycnal mixing, parcel subduction and mean flow instabilities.

The Antarctic Circumpolar Current (ACC) in the Scotia Sea comprises
three distinct fronts; the Subantarctic, Polar and southern ACC
fronts [Orsi et al., 1995]. About 100 Sv of the estimated total
transport though Drake Passage (134 Sv were estimated from direct
measurements during ISOS [Nowlin and Klinck, 1986]) are carried
continuously about Antarctica by these major fronts. Regional
frontal zones like the Scotia and Weddell fronts have also been
identified.

Water characteristics in the WSC region cannot be explained by
lateral mixing of its adjacent waters from the ACC and Weddell
Sea [Whitworth et al., 1994]. Among possible mechanisms which
have been invoked to account for the WSC water are winter convection
[Deacon, 1937], lateral and vertical boundary mixing [Patterson
and Sievers, 1980], injection of meltwater from ice shelves farther
south [Patterson and Sievers, 1980], and advection of shelf waters
which have been conditioned along the eastern side of the Antarctic
Peninsula [Gordon and Nowlin, 1978; Whitworth et al., 1994].
It seems probable that the last is the primary source of WSC water,
but additional field observations and direct current measurements
are needed to test and quantify this mechanism.

Transfer of water northward through the WSC region is, likewise,
not well understood. We hypothesize above that 15 Sv of water
must move north through the WSC in addition to the transport contained
in topographically trapped bottom boundary currents. Available
measurements of currents in the WSC region are inadequate to describe
the regional circulation, however, currents derived using numerical
model results suggest that the mean circulation is primarily zonal
(Figure 2), offering little in the way of a meridional advective
mechanism. Past field work in the region [e.g. Foster and Middleton,
1984; Muench et al., 1990] and instantaneous results from the
Semtner and Chervin [1992] model show energetic mesoscale activity,
however, which might lead to significant meridional transports.

Figure 2. Four year mean currents at 117.5 m (topmost), 1542.5
m (middle) and 3025 m (bottom) derived from the Semtner and Chervin
[1992] model. Antarctic Peninsula (AP) is on the lower left, and
the South Orkney Islands (SOI) are situated near the center of
the figure. The Scotia (SF) and Weddell (WF) fronts show clearly
as bands of stronger zonal north and south, respectively, of the
South Orkneys. The deepest currents show northward flow through
the gap west of the South Orkneys, but meridional flows are in
general weak.

Neutral density surfaces [McDougall, 1987] are close approximations
to isentropic surfaces, thus they can provide a useful approach
to study water mass interactions in the WSC region because they
include thermobaric and other nonlinear effects of the equation
of state. For example, water lying at or near the shelf break
of the northwestern Weddell Sea shelf can be traced on neutral
surfaces to depths greater than 3500 m in the northern Scotia
Sea (Figure 3). In this way, waters from the eastern shelf off
the Antarctic Peninsula can be assumed to ventilate the Georgia
and Scotia basins.

Figure 3. Meridional section showing neutral density surfaces
from 62 to 57.5S across the South Scotia Ridge and demonstrating
the contribution of mid-depth to deep Weddell Sea waters to Antarctic
Bottom Water in the Global Ocean [Orsi, 1995 personal communication].

Tidal currents are a significant component of the total velocity
field in many parts of the Weddell-Scotia Confluence region.
Tides can affect the regional hydrography in several ways which
include increasing the benthic and underice stress, generating
mean lagrangian and eulerian circulations parallel to sloping
topography, and generating baroclinic tidal and other higher frequency
internal gravity waves that then provide energy to enhance diapycnal
mixing rates in the pycnocline. Padman and Dillon [1992] and
Padman [1995] consider some of these processes in the Arctic,
while Foster et al. [1987] consider them for the southern Weddell
Sea.

Each of the tide-related processes has repercussions for the present
proposed study. Increased benthic mixing raises the rate at which
the boundary flows of dense WSBW are modified by entrainment of
the ambient water through which it flows. Mean lagrangian flows
paralleling bottom contours provide a mechanism which, in addition
to vorticity conservation, steers the WSBW around topographic
features. Turbulence associated with internal gravity waves modifies
the diapycnal mixing rate which can then affect the entrainment
rate for the WSBW bottom plumes, mixing in the pycnocline, and
entrainment of the pycnocline by mixed layer turbulence whether
it be driven by shear stress or convection.